Cyanobacterial cultures for the production of biofuels or other products are often grown as an axenic or nearly axenic “monoculture” to improve yields of the product. Once the culture reaches a large scale, the requirement for light for photosynthesis often results in the culture being grown outdoors in non-axenic conditions, such as open ponds, non-sterile tubes, or plastic transparent containers. The cost of keeping a cyanobacterial production culture from becoming contaminated, however, is a major obstacle, particularly for the outdoor, large-scale, commercial production of a product of interest. Although the growth system can be pre-cleaned or pre-sterilized before the cyanobacterial culture is added, it is often difficult to keep contaminants out of the system, particularly during longer culture run times, such as 20 days or more.
The use of various drugs such as antibiotics for the prevention of contamination with microorganisms other than the cyanobacterial production strains can also be used, where the cyanobacterial culture is maintained by introducing an antibiotic-resistance gene into the cyanobacterial host cell and culturing the cells in a culture medium containing an antibiotic corresponding to the antibiotic-resistance gene. However, a culturing method that uses an antibiotic may not be suitable for use at a larger scale.
Cyanobacterial production systems typically utilize culture media that are formulated to provide the nutrients necessary for the growth of a host cell, such as carbon, nitrogen, phosphorus, sulfur, and other major nutrients. Some cell cultures also require additional components, such as trace minerals and metals. These nutrients provide a suitable growth environment for the cyanobacterial strain of choice and, unfortunately, for many potential contaminating organisms. Thus, the cyanobacterial production strain is required to compete directly with contaminant organisms in the cell culture for nutrients.
What is needed in the art is a method of growing cyanobacterial cultures in a way that reduces the growth of contaminating organisms such as heterotrophic bacteria, so that the cyanobacterial cells can grow and produce the product of interest at maximal yields in a production environment.